Laboratory of Chemical Pharmacology, Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. nomura@mol.f.u-tokyo.ac.jp.

Abstract

BACKGROUND:

Prolonged re-exposure to a fear-eliciting cue in the absence of an aversive event extinguishes the fear response to the cue, and has been clinically used as an exposure therapy. Arc (also known as Arg3.1) is implicated in synaptic and experience-dependent plasticity. Arc is regulated by the transcription factor cAMP response element binding protein, which is upregulated with and necessary for fear extinction. Because Arc expression is also activated with fear extinction, we hypothesized that Arc expression is required for fear extinction.

FINDINGS:

Extinction training increased the proportion of Arc-labeled cells in the basolateral amygdala (BLA). Arc was transcribed during latter part of extinction training, which is possibly associated with fear extinction, as well as former part of extinction training. Intra-BLA infusions of Arc antisense oligodeoxynucleotide (ODN) before extinction training impaired long-term but not short-term extinction memory. Intra-BLA infusions of Arc antisense ODN 3 h after extinction training had no effect on fear extinction.

CONCLUSION:

Our findings demonstrate that Arc is required for long-term extinction of conditioned fear and contribute to the understanding of extinction as a therapeutic manner.

Extinction training upregulates Arc expression in the BLA. (A) The Extinction group underwent contextual fear conditioning and extinction training, in which they were re-exposed to the conditioning context for 40 min without shock. The No Extinction group underwent fear conditioning but not extinction training. (B) Representative images of Arc-immunolabeled cells in the Extinction and No Extinction groups, respectively. (C) The Extinction group demonstrated more Arc-labeled cells relative to the No Extinction group in both BA and LA (n = 6 mice per group, **p < 0.01).

Arc is transcribed during both former and latter parts of extinction training. (A) The Extinction group underwent contextual fear conditioning and extinction training, in which they were re-exposed to the conditioning context for 35 min without shock. The No Extinction group underwent fear conditioning but not extinction training. (B) Representative images of Arc-positive cells in the Extinction and No Extinction groups, respectively. (C) The Extinction group demonstrated more cytoplasmic and nuclear Arc-positive cells relative to the No Extinction group in the BLA (Extinction, n = 5 mice; No Extinction, n = 9 mice, **p < 0.01).

Inhibiting Arc translation in the BLA impairs long-term extinction of conditioned fear. (A) Mice received intra-BLA infusions of either Arc antisense or scrambled ODN 3 h before extinction training, and were subjected to short-term (STM) and long-term memory (LTM) tests 2 h and 24 h later, respectively. (B) Freezing time during extinction training decreased over time. The mice that received antisense ODN had a longer freezing period versus the mice that received scrambled ODN in the LTM test, but not the STM test (n = 14 mice per group, **p < 0.01). (C) Histological verification of cannula placements for mice infused with Arc antisense or scrambled ODN.